Make StartUpColorTemperatureMireds nullable, per spec.

[woody-apple]

Fixes #21855

Problem

Attribute is not nullable.

Change overview

It should be.

Testing

Examined the generated code.

@jmartinez-silabs

[github-actions]

PR #22109: Size comparison from bdbcb40 to 1e1ac85

Increases (14 builds for bl602, cc13x2_26x2, efr32, esp32, k32w, nrfconnect)

platform	target	config	section	bdbcb40	1e1ac85	change	% change
bl602	lighting-app	bl602	(read/write)	1383882	1383914	32	0.0
			.text	1051032	1051068	36	0.0
		bl602+rpc	(read/write)	1429394	1429426	32	0.0
			.text	1082792	1082828	36	0.0
cc13x2_26x2	all-clusters-app	LP_CC2652R7	(read only)	672379	672411	32	0.0
			.text	583316	583348	32	0.0
	all-clusters-minimal-app	LP_CC2652R7	(read only)	637763	637779	16	0.0
			.text	559460	559476	16	0.0
	lock-mtd	LP_CC2652R7	(read/write)	180576	180584	8	0.0
	shell	LP_CC2652R7	(read only)	665062	665094	32	0.0
			.text	579052	579084	32	0.0
efr32	lighting-app	BRD4161A	(read/write)	1101124	1101172	48	0.0
			.text	965628	965676	48	0.0
		BRD4161A+rpc	(read/write)	1155392	1155440	48	0.0
			.text	1003000	1003048	48	0.0
		BRD4161A+rs911x	(read/write)	990264	990296	32	0.0
			.text	825460	825492	32	0.0
esp32	all-clusters-app	c3devkit	(read only)	1029982	1030020	38	0.0
			.flash.text	1029982	1030020	38	0.0
		m5stack	(read only)	1083331	1083371	40	0.0
			.flash.text	1077947	1077987	40	0.0
k32w	light	k32w0+release	(read/write)	645476	645508	32	0.0
			.text	570472	570504	32	0.0
nrfconnect	all-clusters-app	nrf52840dk_nrf52840	(read/write)	1180067	1180099	32	0.0
			text	814084	814116	32	0.0
	all-clusters-minimal-app	nrf52840dk_nrf52840	(read/write)	1159927	1159959	32	0.0
			text	803072	803104	32	0.0

Decreases (3 builds for cc13x2_26x2)

platform	target	config	section	bdbcb40	1e1ac85	change	% change
cc13x2_26x2	all-clusters-app	LP_CC2652R7	(read/write)	179132	179100	-32	-0.0
	lock-mtd	LP_CC2652R7	(read only)	656839	656831	-8	-0.0
			.text	554600	554592	-8	-0.0
	shell	LP_CC2652R7	(read/write)	181968	181936	-32	-0.0

Full report (21 builds for bl602, cc13x2_26x2, efr32, esp32, k32w, mbed, nrfconnect)

platform	target	config	section	bdbcb40	1e1ac85	change	% change
bl602	lighting-app	bl602	(read/write)	1383882	1383914	32	0.0
			.bss	120290	120290	0	0.0
			.data	4480	4480	0	0.0
			.text	1051032	1051068	36	0.0
		bl602+rpc	(read/write)	1429394	1429426	32	0.0
			.bss	127730	127730	0	0.0
			.data	4600	4600	0	0.0
			.text	1082792	1082828	36	0.0
cc13x2_26x2	all-clusters-app	LP_CC2652R7	(read only)	672379	672411	32	0.0
			(read/write)	179132	179100	-32	-0.0
			.bss	74404	74404	0	0.0
			.data	3372	3372	0	0.0
			.rodata	88747	88747	0	0.0
			.text	583316	583348	32	0.0
	all-clusters-minimal-app	LP_CC2652R7	(read only)	637763	637779	16	0.0
			(read/write)	157980	157980	0	0.0
			.bss	73692	73692	0	0.0
			.data	3372	3372	0	0.0
			.rodata	77979	77979	0	0.0
			.text	559460	559476	16	0.0
	lock-ftd	LP_CC2652R7	(read only)	674095	674095	0	0.0
			(read/write)	167632	167632	0	0.0
			.bss	71508	71508	0	0.0
			.data	3296	3296	0	0.0
			.rodata	76671	76671	0	0.0
			.text	596944	596944	0	0.0
	lock-mtd	LP_CC2652R7	(read only)	656839	656831	-8	-0.0
			(read/write)	180576	180584	8	0.0
			.bss	67196	67196	0	0.0
			.data	3296	3296	0	0.0
			.rodata	101759	101759	0	0.0
			.text	554600	554592	-8	-0.0
	pump-app	LP_CC2652R7	(read only)	684767	684767	0	0.0
			(read/write)	157768	157768	0	0.0
			.bss	71548	71548	0	0.0
			.data	3296	3296	0	0.0
			.rodata	89959	89959	0	0.0
			.text	594324	594324	0	0.0
	pump-controller-app	LP_CC2652R7	(read only)	669259	669259	0	0.0
			(read/write)	173396	173396	0	0.0
			.bss	71668	71668	0	0.0
			.data	3292	3292	0	0.0
			.rodata	85515	85515	0	0.0
			.text	583264	583264	0	0.0
	shell	LP_CC2652R7	(read only)	665062	665094	32	0.0
			(read/write)	181968	181936	-32	-0.0
			.bss	76724	76724	0	0.0
			.data	3376	3376	0	0.0
			.rodata	85694	85694	0	0.0
			.text	579052	579084	32	0.0
efr32	lighting-app	BRD4161A	(read/write)	1101124	1101172	48	0.0
			.bss	133404	133404	0	0.0
			.data	2068	2068	0	0.0
			.text	965628	965676	48	0.0
		BRD4161A+rpc	(read/write)	1155392	1155440	48	0.0
			.bss	150092	150092	0	0.0
			.data	2280	2280	0	0.0
			.text	1003000	1003048	48	0.0
		BRD4161A+rs911x	(read/write)	990264	990296	32	0.0
			.bss	162728	162728	0	0.0
			.data	2056	2056	0	0.0
			.text	825460	825492	32	0.0
	lock-app	BRD4161A+wf200	(read/write)	1139812	1139812	0	0.0
			.bss	145904	145904	0	0.0
			.data	2064	2064	0	0.0
			.text	991824	991824	0	0.0
	window-app	BRD4161A	(read/write)	1092580	1092580	0	0.0
			.bss	134844	134844	0	0.0
			.data	2096	2096	0	0.0
			.text	955620	955620	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	1029982	1030020	38	0.0
			(read/write)	1488674	1488674	0	0.0
			.dram0.bss	70864	70864	0	0.0
			.dram0.data	14600	14600	0	0.0
			.flash.rodata	217776	217776	0	0.0
			.flash.text	1029982	1030020	38	0.0
			.iram0.text	62902	62902	0	0.0
		m5stack	(read only)	1083331	1083371	40	0.0
			(read/write)	490632	490632	0	0.0
			.dram0.bss	76376	76376	0	0.0
			.dram0.data	34144	34144	0	0.0
			.flash.rodata	248116	248116	0	0.0
			.flash.text	1077947	1077987	40	0.0
			.iram0.text	123267	123267	0	0.0
k32w	light	k32w0+release	(read/write)	645476	645508	32	0.0
			.bss	70232	70232	0	0.0
			.data	2044	2044	0	0.0
			.text	570472	570504	32	0.0
	lock	k32w0+release	(read/write)	703028	703028	0	0.0
			.bss	70704	70704	0	0.0
			.data	2052	2052	0	0.0
			.text	627544	627544	0	0.0
mbed	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2454328	2454328	0	0.0
			.bss	215076	215076	0	0.0
			.data	5872	5872	0	0.0
			.text	1416972	1416972	0	0.0
nrfconnect	all-clusters-app	nrf52840dk_nrf52840	(read/write)	1180067	1180099	32	0.0
			bss	143759	143759	0	0.0
			rodata	143268	143268	0	0.0
			text	814084	814116	32	0.0
	all-clusters-minimal-app	nrf52840dk_nrf52840	(read/write)	1159927	1159959	32	0.0
			bss	142996	142996	0	0.0
			rodata	134944	134944	0	0.0
			text	803072	803104	32	0.0